The present invention relates to a macrolens system. More particularly, the present invention relates to a macrolens system having a long focal length. More specifically, it relates to a macrolens system which can photograph an object from an infinitely far distance to a close distance without substantially changing its overall length.
In photographing an object which has a magnification of about 1.times., it is well known to use a technique of moving the focusing lens group using a bellows. However, it is difficult to operate a bellows and it is not compact. Such method also decreases image quality at a short taking distance significantly, providing poor imaging performance.
In recent years, along with the popularity of the auto focus (AF) camera, demands have been made that the amount of work be reduced; that is the work is defined as the weight of a focusing lens group multiplied by the amount of movement. There are some proposals which work with auto focusing operations and, at the same time, prevent the focusing lens group from deteriorating its imaging performance by working on the focusing method in various ways.
Particularly, in a macrolens system which has a long focal length, the amount which the focusing lens group moves from focusing on an infinitely far object to focusing on a close object tends to be undesirably large. Therefore, an optical system having a configuration which does not impede the AF operation and the method of focusing needs to be developed. Such a macrolens system, which is able to photograph an object at an infinitely far distance and also photograph an object at a close distance, such as at 1.times. is disclosed for example in Japanese Patent Publication No. S62-42252 and Japanese Laid-Open Patent Aplication No. H2-81014, S55-140810, S61-132916 and H5-32319.
Also, in Japanese Patent Publication No. S55-78326, H5-42643, and H 6-46259, etc., for example, is disclosed a "long focal length lens system" in which the distance between the end of the lens closest to the object and the image plane of the lens system, that is the overall length, does not change. However, the long focal length lens disclosed in these patent publications is able to photograph at 1/7.times., 1/10.times., and 1/4.times. only. This does not fully achieve the desirable functions of a long focal length macrolens system.
In the long focal length macrolens system disclosed in Japanese Patent Publication No. S62-42252, the first lens group is fixed. This patent publication is assigned to the same assignee as the present invention. This arrangement provides an attractive advantage that the overall length of the system does not change. However the astigmatism fluctuates greatly along with the change in focal length from infinitely far distances to 1.times.. Also, a great degree of aberration is generated at a mid-range objective focal length state.
In the same manner, in the long focal length macrolens system disclosed in Japanese Laid-Open Patent Application H2-81014 by the same assignee as this invention, almost all of the problems S62-42252 had are resolved including the fact that the overall length of the macrolens system does not change. However, the overall macrolens system is long. Its spherical aberration fluctuates and its distortion at the 1.times. focal state is undesirably large.
On the other hand, in the long focal length macrolens system disclosed in Japanese Laid-Open Patent Publication No. S55-140810, the change of total length is large, and the work of the focusing lens group is large. The fluctuation of spherical aberration is unfavorably large as well.
In the long focal length macrolens system disclosed in Japanese Laid-Open Patent Publication No. S61-132916, the aberration fluctuation along with the change in the object distance is small. However, the number of lenses which constitute this system is large. This system also moves the first lens group which is heavy and bulky. This increases the overall length during focusing from infinitely far to a near distance. The center of gravity of the lens system moves a large distance and the amount of work used in moving the focusing lens group is unfavorably large.
Also, in the long focal length macrolens system disclosed in Japanese Laid-Open Patent Publication No. H5-323191 by the same assignee as the present invention, the large amount of movement of the center of gravity of the lens system due to the change in objective distance can be suppressed. The aberration fluctuation is best suppressed, of all the above related arts. However the first lens group is moved in this technique, thus poorly suppressing the spherical aberration.
The long focal length macrolens system disclosed in Japanese Laid-Open Publication No. H5-323191 resolved some problems such as:
a fluctuation of astigmatism and overcompensation of the spherical aberration. These problems were corrected by moving the first lens group to the object-side. However, moving the first lens group, which is the bulkiest, is undesirable regardless of the amount of movement, in terms of operability and manufacturability. In other words, if the first lens group is fixed and yet the overall aberration can be balanced, an easy-to-operate, easy-to-manufacture, long focal length macrolens system of consistent quality can be obtained for reduced cost.